Etching methods, methods of removing portions of material, and methods of forming silicon nitride spacers

ABSTRACT

In one aspect, the invention includes a method of removing at least a portion of a material from a substrate, comprising: a) first etching the material in a reaction chamber; b) second etching the material in the reaction chamber; and c) cleaning a component of the material from at least one sidewall of the reaction chamber between the first etching and the second etching. In another aspect, the invention includes a method of patterning a material over a semiconductive substrate, comprising: a) forming a layer of first material against a second material and over a semiconductive substrate, the semiconductive substrate comprising a surface having a center and an edge; b) first etching the first material in a reaction chamber, the first etching comprising a first center-to-edge uniformity across the surface of the wafer and comprising a first selectivity for the first material relative to the second material; c) second etching the first material in the reaction chamber, the second etching comprising a second center-to-edge uniformity across the surface of the wafer and comprising a second selectivity for the first material relative to the second material, the second center-to-edge uniformity being less than the first center-to-edge uniformity, the second selectivity being greater than the first selectivity; and d) cleaning a component of the first material from at least one sidewall of the reaction chamber between the first etching and the second etching.

TECHNICAL FIELD

[0001] The invention pertains to etching methods, such as, for example,methods of forming silicon nitride spacers.

BACKGROUND OF THE INVENTION

[0002] A commonly utilized method for removing at least some of amaterial is plasma etching. Such method can be used, for example, insemiconductor processing. An enormous diversity of materials can beremoved by appropriately adjusting etchant components and etchingparameters. Among the materials that can be removed are polycrystallinesilicon, silicon nitride and silicon oxides. Etchants that can beutilized for removing polycrystalline silicon include HCl, HBr, HI, andCl₂, alone or in combination with each other and/or one or more of He,Ar, Xe, N₂, and O₂. A suitable etchant that can be utilized for removinga silicon oxide, such as silicon dioxide, is a plasma comprisingCF₄/CHF₃, or CF₄/CH₂F₂. Additionally, a suitable etchant for removingsilicon oxide is a plasma comprising a large amount of CF₃, and a minoramount of CH₂F₃. A suitable etchant for removing silicon nitride is aplasma comprising CF₄/HBr.

[0003] An example prior art reaction vessel 10 is illustrated in FIG. 1.Reaction vessel 10 comprises a plurality of sidewalls 12 surrounding aninternal reaction chamber 14. Also, reaction vessel 10 comprises a radiofrequency (RF) generating coil 16 surrounding a portion of reactionchamber 14 and connected to a first RF source 18. RF coil 16 isconfigured to generate a plasma within reaction chamber 14.

[0004] A substrate 20 is received within internal chamber 14 andconnected to a second RF source 22. Second RF source 22 is configured togenerate an RF bias at substrate 20. Additionally, reaction vessel 10can comprise coolant coils (not shown) configured to cool a backside ofsubstrate 20 and thereby maintain substrate 20 at a desired temperatureduring an etching process. It is to be understood that vessel 10 is anexemplary etching vessel. Other constructions are possible. Forinstance, reaction vessel 10 utilizes a cylindrical inductively drivensource geometry, but planar or other inductively driven sourcegeometries can be used. Also, reaction vessel 10 is shown utilizing twoseparate RF sources, 18 and 20, but other constructions can be usedwherein a single RF source can be utilized and the RF power from suchsource split to form a first RF power at coil 16 and an RF bias atsubstrate 20.

[0005] Substrate 20 can comprise, for example, a monocrystalline siliconwafer. To aid in interpretation of the claims that follow, the term“semiconductive substrate” is defined to mean any constructioncomprising semiconductive material, including, but not limited to, bulksemiconductive materials such as a semiconductive wafer (either alone orin assemblies comprising other materials thereon), and semiconductivematerial layers (either alone or in assemblies comprising othermaterials). The term “substrate” refers to any supporting structure,including, but not limited to, the semiconductive substrates describedabove.

[0006] In operation, plasma gases (not shown) are flowed into internalchamber 14 and converted into a plasma by energy input from reactioncoil 16. An RF bias is generated at substrate 20, and such RF bias drawsplasma components to a surface of substrate 20 to etch a material atsuch surface.

[0007] During etching of a component from substrate 20, the materialsproduced by chemical reaction to the substrate with etch gases arereleased into the internal chamber. Such materials are referred toherein as etch reaction products, or as etchant debris. A method ofdetermining when an etch has penetrated a material is to monitor theconcentration of the evolved reaction products and/or etchant gases asthe etch proceeds. Monitoring of the etchant debris can be accomplishedby, for example, spectroscopic methods, including, for example,ultraviolet-visible spectroscopy and mass spectrometry. Preferably, themonitoring will be performed by an automated system, with softwareconfigured to detect when a concentration of a monitored materialdecreases within the etchant debris.

[0008] In the shown embodiment, a monitoring device 28 is provided toobserve etchant debris within reaction chamber 14 through a window 26.Monitoring device 28 can comprise, for example, a spectrometer. Thespectrometer can be configured to, for example, display a signalcorresponding to a concentration of a particular component in theetchant debris, and/or to send such signal to an automated mechanismwhich performs a function in response to particular signalcharacteristics. An example automated system is a system comprising analgorithm to analyze the signal and determine from the analysis when anetch penetrates a particular material. The automated system can beconfigured to terminate the etching process in response to adetermination that the etch has penetrated the particular material.

[0009] An etch will frequently be conducted in two distinct etchingsteps, particularly if the etching is to remove a thickness of materialthat is greater than or equal to 200 Angstroms. First, a highly physical(non-selective) etch is utilized to etch through the majority of amaterial. Second, a chemical-type etch (highly selective) is utilized toetch through a remainder of the material. A less selective(physical-type) etch generally has better center-to-edge uniformity thana more selective (chemical-type) etch. Center-to-edge uniformity can beunderstood by reference to FIG. 2 wherein a semiconductive wafer 40 isillustrated. Wafer 40 comprises an edge region 42 and a center region44. Generally, an etch process will etch material from both edge region42 and center region 44, as well as from regions intermediate edgeregion 42 and center region 44. Etching frequently progresses at adifferent rate at edge region 42 than at center region 44. Thus, as anetch progresses further into a material of semiconductive wafer 40, adisparity between etchant depth at center region 44 and edge region 42becomes more pronounced. Center-to-edge uniformity is a measure of adegree of disparity between an etch rate at edge region 42 versus anetch rate at center region 44.

[0010] Physical-type etch processes generally have a high degree ofcenter-to-edge uniformity, and therefore etch edge region 42 at aboutthe same rate as center region 44. In contrast, chemical-type edgestypically have a lower degree of center-to-edge uniformity, andaccordingly etch edge region 42 at a significantly different rate thancenter region 44.

[0011] A reason for utilizing a physical-type etch initially in anetching process is to maintain a high degree of center-to-edgeuniformity as the bulk of a material is etched. The etching process isthen changed to a more chemical-type etch as a final portion of thematerial is removed to obtain a high degree of selectivity for thematerial relative to other materials that can be exposed during latterstages of an etch.

[0012] A chemical-type etch and a physical-type etch can utilize thesame etchants but vary in power settings and pressures, or can utilizedifferent etchants at either the same or different power settings andpressures. If the physical-type etch and chemical-type etch comprise thesame etchants, the physical-type etch generally comprises a higher biaspower at a substrate, and a lower pressure within a reactor than thechemical-type etch. For example, both chemical-type etching andphysical-type etching of a silicon nitride material can utilize anetchant comprising CF₄/HBr. However, the physical-type etching willutilize an RF power to primary RF coil 16 of from about 250 to about 800watts, a bias power to substrate 20 of from about 75 to about 400 watts,and a pressure within internal chamber 14 of from about 5 to about 15mTorr. In contrast, a chemical-type etch will utilize a power to primaryRF coil 16 (FIG. 1) of from about 300 to about 900 watts, a bias powerto substrate 20 of less than about 20 watts, and a pressure withininternal chamber 14 of from about 40 to about 70 mTorr.

[0013] A difficulty in etching methods can occur during monitoring ofetchant debris. For instance, a nitride spacer etch is described withreference, to FIGS. 3-5, with a semiconductor wafer fragment 50illustrated before an etch (FIG. 4) and after the etch (FIG. 5), and agraph of nitrogen-containing components in debris from the etch shown inFIG. 3. In the before-etch-construction of FIG. 4, wafer fragment 50comprises a substrate 52 having a transistor gate construction 54 formedthereover. Substrate 52 can comprise, for example, monocrystallinesilicon lightly doped with a P-type dopant. Transistor gate structure 54comprises a silicon dioxide layer 56, a polycrystalline silicon layer58, a metal-silicide layer 60, and an insulative cap 62. Metal-silicidelayer 60 can comprise, for example, titanium-silicide ortungsten-silicide, and insulative cap 62 can comprise, for example,silicon dioxide or silicon nitride. In the shown construction, silicondioxide layer 56 extends beyond lateral peripheries of gate construction54 and over an upper surface of substrate 52. A silicon nitride layer 64is formed over silicon dioxide layer 56, as, well as over gate structure54. In other constructions (not shown) an extent of silicon dioxidelayer 56 can be limited to within the lateral peripheries of gateconstruction 54, and silicon nitride layer 64 can contact substrate 52in regions beyond the lateral peripheries of gate construction 54.

[0014] Referring to FIG. 5, an etch is conducted to pattern siliconnitride layer 64 into sidewall spacers 66. The etching has selectivelystopped at oxide layer 56. Preferably, insulative cap 62 comprisessilicon dioxide so that the etch of nitride layer 64 also selectivelystops at cap 62.

[0015] The etch of silicon nitride layer 64 comprises two distinct etchsteps, an initial physical-type etch, and a subsequent chemical-typeetch. The FIG. 3 graph of nitrogen composition in etchant debris,illustrates the intensity of a 386 nanometer signal obtained as afunction of time. The 386 nanometer signal is associated with a C-Nexcitation. The physical-type etch forms a first peak region 70 ofnitrogen-containing material in the etch debris, and the chemical-type,etch forms a second peak region 72 of nitrogen-containing material inthe etch debris. A trough region 74 occurs between peak regions 70 and72, and corresponds to a period of time wherein etching conditionswithin the reaction chamber are switched from physical-type etchingconditions to chemical-type etching conditions.

[0016] A difficulty occurs in monitoring peak region 72 to ascertain theprecise time at which nitride layer 64 (FIGS. 4 and 5) has been etchedthrough to oxide layer 56 (FIGS. 4 and 5). Careful observation of peakregion 72 reveals a break at a location labeled 76. Such breakcorresponds to a significant drop in nitrogen-containing species withinan etch debris, and corresponds to the time at which the shown etch haspenetrated silicon nitride layer 64. Although the break at location 76can be discerned by a person viewing peak region 72, it is difficult tocreate software algorithms that can accurately detect break 76 on theoverall peak-shape of peak region 72. Specifically, peak region 72comprises a sloped trailing edge before the drop in nitrogen speciesoccurring at location 76. Such sloped trailing edge effectively createsa sloping baseline upon which location 76 is to be identified. It isdifficult to create software algorithms that can reproducibly discern achange on a sloping baseline. Accordingly, it is desirable to developmethods for substantially removing the sloping trailing edge of peakregion 72.

SUMMARY OF THE INVENTION

[0017] In one aspect, the invention encompasses a method of removing atleast a portion of a material from a substrate. The material issubjected to a first etching and a second etching in a reaction chamber.A component of the material is removed from at least one sidewall of thereaction chamber between the first etching and the second etching.

[0018] In another aspect, the invention encompasses a method of 8patterning a material over a semiconductive substrate. A layer of firstmaterial is formed against a second material and over a semiconductivesubstrate. The semiconductive substrate comprises a surface having acenter and an edge. The first material is subjected to first etching ina reaction chamber. The first etching comprises a first center-to-edgeuniformity across the surface of the wafer and comprises a firstselectivity for the first material relative to the second material. Thefirst material is subjected to second etching in the reaction chamber.The second etching comprises a second center-to-edge uniformity acrossthe surface of the wafer and comprises a second selectivity for thefirst material relative to the second material. The secondcenter-to-edge uniformity is less than the first center-to-edgeuniformity, and the second selectivity is greater than the firstselectivity. A component of the first material is cleaned from at leastone sidewall of the reaction chamber between the first etching and thesecond etching.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Preferred embodiments of the invention are described below withreference to the following accompanying drawings.

[0020]FIG. 1 is a schematic, cross-sectional side-view of a prior artplasma-etching chamber.

[0021]FIG. 2 is a schematic top view of a prior art wafer fragment.

[0022]FIG. 3 is a graph of intensity vs. time of a 386 nanometer signalin a prior art etching process.

[0023]FIG. 4 is a fragmentary, schematic, cross-sectional side-view of asemiconductor wafer fragment at a preliminary processing step of a priorprocessing method.

[0024]FIG. 5 is a view of the FIG. 4 wafer fragment shown at a prior artprocessing step subsequent to that of FIG. 4.

[0025]FIG. 6 is a graph of intensity vs. time of a 386 nanometer signalin an etching process conducted in accordance with a method of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026] This disclosure of the invention is submitted in furtherance ofthe constitutional purposes of the U.S. Patent Laws “to promote theprogress of science and useful arts” (Article 1, Section 8).

[0027] It is observed that the sloped trailing edge of prior art peakregion 72 (FIG. 3) indicates that there is a diminishing amount ofnitrogen containing species within a prior art etching chamber, evenbefore the drop in nitrogen species occurring at location 76. It is alsoobserved that such diminishing amount of nitrogen-containing species canbe due to nitrogen-containing components being inadvertently displacedfrom sidewalls of a reaction chamber and into an etch debris during theetch of silicon nitride layer 64 (FIG. 4). One or more of severalmechanisms could occur to deposit nitrogen-containing components onsidewalls of a reaction chamber. The nitrogen-containing components canbe deposited on the sidewalls, for example, during either or both of aphysical-type etch and a chemical-type etch of a silicon nitridematerial.

[0028] In accordance with the present invention, a sidewall of thereaction chamber is cleaned prior to an etching of a material within thechamber to reduce or eliminate inadvertent release of components fromthe sidewalls during the etching of the material. In a preferredembodiment of the invention, a sidewall of a reaction chamber is cleanedafter a physical-type etch of a material within the chamber, and beforea chemical-type etch of the material within the same chamber.

[0029] A sidewall of a reaction chamber can be cleaned by a number ofmethods. For instance, the sidewall can be subjected to a plasmacontaining one or more strong oxidants. Suitable plasmas can include,for example, SF₆, Cl₂ or NF₃ in combination with oxygen atoms. Morespecifically, suitable plasmas can include, for example, SF₆/O₂, Cl₂/O₂,or NF₃/O₃. Suitable etch conditions can comprise, for example, 500 wattsof power to a primary RF coil (such as RF coil 16 of the prior artreactor construction of FIG. 1) and from about 20 to about 60 mTorr ofpressure within a reaction chamber. Preferably, a substrate will remainin the reaction chamber during cleaning of the sidewalls, but no biaspower will be applied to the substrate. As no bias power is applied tothe substrate, the oxidizing plasma within the chamber is substantiallykept from etching materials on the substrate. In other words, theoxidizing plasma substantially selectively removes materials fromsidewalls of the reaction chamber and not from the substrate. In thecontext of this document, the term “substantially selectively” meansthat the oxidizing plasma removes materials from the sidewalls of thereaction chamber at a rate that is at least 2 times greater than a rateat which materials are removed from the substrate, and preferably atleast 10 times greater. As the debris film on a chamber wall istypically very thin (frequently less than 20 Angstroms), the oxidizingplasma treatment of the chamber sidewalls is for a brief enough timeperiod that typically less than 5 Angstroms of material is removed froma wafer in the chamber during the oxidizing plasma treatment.

[0030] It is found that cleaning of the sidewalls of a reaction chamberbetween a physical-type etch and a chemical-type etch can alleviate themonitoring problems discussed above in the “background” section of thisdisclosure. A graph of nitrogen-component concentration versus time fora silicon nitride etch process conducted in accordance with the presentinvention is shown in FIG. 6. More specifically, FIG. 6 is a graph ofsignal intensity at 386 nanometers for a silicon nitride spacer etch,such as the etch described above with reference to FIGS. 4 and 5.Accordingly, the graph of FIG. 6 corresponds to similar processing asthat described with reference to the prior art graph of FIG. 3, with adifference that the processing of FIG. 6 incorporates a sidewallcleaning step between a physical-type etch and a chemical-type etch.

[0031] The FIG. 6 the graph comprises three distinct peak regions, 90,92 and 94. Peak region 90 corresponds to nitrogen-comprising componentsreleased during a physical-type etch of silicon nitride, and peak region92 corresponds to nitrogen-components released during a chemical-typeetch of the silicon nitride. Accordingly, peak regions 90 and 92correspond to similar etches as peak regions 70 and 72 of the prior artgraph shown in FIG. 3.

[0032] A difference between the graph of FIG. 6 and that of FIG. 3 isthat peak region 92 of the chemical-type etch after a cleaning step ofthe present invention has a much flatter upper surface than does peakregion 72 of the prior art chemical-type etch. Accordingly, peak region92 provides a relatively flat baseline against which a decrease innitrogen component concentration of etchant debris can be ascertained.For instance, peak region 92 contains an easily, identified location 96wherein a nitrogen-component concentration in an etch debris isdecreasing. A comparison of peak regions 92 and 72 indicates that adecrease in nitrogen component concentration of etchant debris issignificantly easier to detect with peak region 92 than with prior artpeak region 72. Such easier detection can aid automated detectionmechanisms. For instance, location 96 can be readily recognized even byconventional software algorithms as a distinct drop in intensityrelative to the flat baseline at the top of peak region 92. Even inprocesses wherein automated detection mechanisms are not utilized, peakregion 92 can have significant advantages relative to peak region 72.For instance, in manual operations (wherein a human operator isdetecting a decrease in an etch debris component concentration) theoperator can more readily detect the concentration change relative topeak region 92 than relative to prior art peak region 72.

[0033] The third peak region of the graph of FIG. 6 (peak region 94)corresponds to nitrogen-components released from the sidewalls of areaction chamber during a cleaning step of the present invention. In theshown preferred embodiment, such cleaning step has occurred between aphysical-type etch (indicated by peak region 90) and a chemical-typeetch (indicated by peak region 92).

[0034] A comparison of the graph of a process of the present inventionin FIG. 6 with the graph of a prior art process in FIG. 3 indicates thatthe present invention release of nitrogen-components by sidewallcleaning, and removal of such components from reaction chamber prior tothe chemical-type etch, can alleviate complications that were occurringin prior art etch monitoring processes.

[0035] Although the process described above is described primarily withreference to a method of etching silicon nitride, it is to be understoodthat the invention has application to many other etch processes. Forinstance, the invention can be utilized during etching of silicon oxidesor other materials. In such etching, a monitored component can be eitheroxygen or silicon. Additionally, the invention can be utilized duringetching of materials that can consist essentially of silicon, such as,for example, polycrystalline silicon, amorphous silicon ormonocrystalline silicon. The silicon can be removed from reactionchamber sidewalls by the strongly oxidizing plasmas described above.

[0036] In compliance with the statute, the invention has been describedin language more or less specific as to structural and methodicalfeatures. It is to be understood, however, that the invention is notlimited to the specific features shown and described, since the meansherein disclosed comprise preferred forms of putting the invention intoeffect. The invention is, therefore, claimed in any of its forms ormodifications within the proper scope of the appended claimsappropriately interpreted in accordance with the doctrine ofequivalents.

1. An etching method, comprising: providing a substrate in a reactionchamber; while the substrate is within the reaction chamber, providing aplasma within the reaction chamber, the plasma substantially selectivelyetching material from the sidewall relative to the substrate; andapplying no bias power to the substrate during the etching.
 2. Anetching method, comprising: providing a substrate in a reaction chamber;providing first etching conditions within the reaction chamber to removea first portion of the substrate; providing second etching conditionswithin the reaction chamber to remove a second portion of the substrate;and cleaning a component from at least one sidewall of the reactionchamber between the first etching and the second etching.
 3. The methodof claim 2 wherein the substrate remains in the reaction chamber duringthe cleaning.
 4. The method of claim 2 wherein the first portion and thesecond portion comprise a common material.
 5. A method of removing atleast a portion of a material from a substrate, comprising: providing asubstrate in a reaction chamber, the substrate having a materialthereover; first etching the material while the substrate is in thereaction chamber; second etching the material while the substrate is inthe reaction chamber; and cleaning a component from at least onesidewall of the reaction chamber between the first etching and thesecond etching, the component comprising a species that is present inthe material.
 6. The method of claim 5 wherein the material comprisessilicon nitride and the species comprised by the component is nitrogen.7. The method of claim 5 wherein the material comprises silicon oxideand the species comprised by the component is silicon.
 8. The method ofclaim 5 wherein the material consists essentially of silicon and thespecies comprised by the component is silicon.
 9. The method of claim 5wherein the substrate remains in the reaction chamber during thecleaning.
 10. The method of claim 5 wherein the first etching comprisesa different pressure than the second etching.
 11. The method of claim 5wherein the substrate is provided with a different bias power during thefirst etching than during the second etching.
 12. The method of claim 5wherein the first etching utilizes a first plasma and the second etchingutilizes a second plasma, the first and second plasmas having differentchemical compositions from one another.
 13. The method of claim 5wherein the first and second etchings pattern the material on thesubstrate.
 14. The method of claim 5 wherein the cleaning comprisesexposing the sidewall to a plasma comprising SF₆ and oxygen atoms. 15.The method of claim 5 wherein the cleaning comprises: exposing thesidewall to a plasma comprising SF₆ and oxygen atoms; maintaining apressure within the reaction chamber at from about 20 mTorr to about 60mTorr; and maintaining the substrate at a bias power of
 0. 16. Themethod of claim 5 wherein the cleaning comprises exposing the sidewallto a plasma comprising chlorine atoms and oxygen atoms.
 17. The methodof claim 5 wherein the cleaning comprises: exposing the sidewall to aplasma comprising chlorine atoms and oxygen atoms; maintaining apressure within the reaction chamber at from about 20 mTorr to about 60mTorr; and maintaining the substrate at a bias power of
 0. 18. Themethod of claim 5 wherein the cleaning comprises exposing the sidewallto a plasma comprising NF₃ and oxygen atoms.
 19. The method of claim 5wherein the cleaning comprises: exposing the sidewall to a plasmacomprising NF₃ and oxygen atoms; maintaining a pressure within thereaction chamber at from about 20 mTorr to about 60 mTorr; andmaintaining the substrate at a bias power of
 0. 20. A method ofpatterning a material over a semiconductive substrate, comprising:forming a layer of first material against a second material and over asemiconductive substrate, the semiconductive substrate comprising asurface having a center and an edge; first etching the first material ina reaction chamber, the first etching comprising a first center-to-edgeuniformity across the surface of the wafer and comprising a firstselectivity for the first material relative to the second material;second etching the first material in the reaction chamber, the secondetching comprising a second center-to-edge uniformity across the surfaceof the wafer and comprising a second selectivity for the first materialrelative to the second material, the second center-to-edge uniformitybeing less than the first center-to-edge uniformity, the secondselectivity being greater than the first selectivity; and cleaning acomponent of the first material from at least one sidewall of thereaction chamber between the first etching and the second etching. 21.The method of claim 20 wherein the semiconductive substrate comprisesmonocrystalline silicon and the second material is monocrystallinesilicon of the semiconductive substrate.
 22. The method of claim 20wherein the semiconductive substrate comprises monocrystalline siliconand the second material is not monocrystalline silicon.
 23. The methodof claim 20 wherein the second etching creates a debris, the methodfurther comprising monitoring the debris for the component to determinewhen the second etching has penetrated the first material.
 24. Themethod of claim 23 wherein the monitoring and determining areaccomplished entirely by an automated mechanism, the automated mechanismcomprising software configured to recognize a drop in a componentconcentration in the debris.
 25. The method of claim 20 wherein thefirst material is over the second material, wherein the second materialdoes not comprise the component, and wherein the second etching createsa debris, the method further comprising monitoring the debris for thecomponent to determine when the second etching has penetrated the firstmaterial to reach the second material.
 26. A method of forming siliconnitride spacers, comprising: forming a transistor gate assembly over asemiconductive substrate, the semiconductive substrate comprising asurface having a center and an edge; forming a silicon dioxide layerproximate the transistor gate assembly; forming a silicon nitride layerover the transistor gate assembly and over the silicon dioxide layer;first etching the silicon nitride in a reaction chamber, the firstetching comprising a first center-to-edge uniformity across the surfaceof the wafer and comprising a first selectivity for the silicon nitriderelative to the silicon oxide; second etching the silicon nitride in thereaction chamber, the second etching comprising a second center-to-edgeuniformity across the surface of the wafer and comprising a secondselectivity for the silicon nitride relative to the silicon oxide, thesecond center-to-edge uniformity being less than the firstcenter-to-edge uniformity, the second selectivity being greater than thefirst selectivity, the first and second etchings patterning the siliconnitride into spacers proximate the transistor gate; and cleaning acomponent of the silicon nitride from at least one sidewall of thereaction chamber between the first etching and the second etching. 27.The method of claim 26 wherein the component comprises nitrogen.
 28. Themethod of claim 26 wherein the substrate remains in the reaction chamberduring the cleaning.
 29. The method of claim 26 wherein the firstetching comprises a different pressure than the second etching.
 30. Themethod of claim 26 wherein the substrate is provided with a differentbias power during the first etching than during the second etching. 31.The method of claim 26 wherein the cleaning comprises exposing thesidewall to a plasma comprising SF₆ and oxygen atoms.
 32. The method ofclaim 26 wherein the cleaning comprises: exposing the sidewall to aplasma comprising SF₆ and oxygen atoms; maintaining a pressure withinthe reaction chamber at from about 20 mTorr to about 60 mTorr; andmaintaining the substrate at a bias power of
 0. 33. The method of claim26 wherein the cleaning comprises exposing the sidewall to a plasmacomprising chlorine atoms and oxygen atoms.
 34. The method of claim 26wherein the cleaning comprises: exposing the sidewall to a plasmacomprising chlorine atoms and oxygen atoms; maintaining a pressurewithin the reaction chamber at from about 20 mTorr to about 60 mTorr;and maintaining the substrate at a bias power of
 0. 35. The method ofclaim 26 wherein the cleaning comprises exposing the sidewall to aplasma comprising NF₃ and oxygen atoms.
 36. The method of claim 26wherein the cleaning comprises: exposing the sidewall to a plasmacomprising NF₃ and oxygen atoms; maintaining a pressure within thereaction chamber at from about 20 mTorr to about 60 mTorr; andmaintaining the substrate at a bias power of
 0. 37. The method of claim26 wherein the second etching creates a debris, the method furthercomprising monitoring the debris for the component to determine when thesecond etching has penetrated the silicon nitride.
 38. The method ofclaim 37 wherein the monitoring and determining are accomplishedentirely by an automated mechanism, the automated mechanism comprisingsoftware configured to recognize a drop in a component concentration inthe debris.